Imaging screens such as those used in rear projection typically have a diffusive element that has an undesirable level of backscatter that reduces contrast, especially in high ambient light situations. Traditional rear projection screens utilizing lenticular lenses and a black stripe material utilize a diffusion layer on the viewer side of the black stripes in order to increase the vertical and horizontal viewing angles. The methods for diffusion of the light include surface relief or volumetric diffusers. These diffusive elements significantly scatter ambient light back towards the viewer which reduces the contrast of the image. Such diffusion layers cannot be placed behind the black stripes because to do so would reduce the resolution of the imaging process used to produce those black stripes. The manufacturing processes typically involve UV curing or ablation by exposing through the lenticular. To place the diffusion layer behind the black stripe would have the effect of increasing the minimum achievable aperture, and in turn the maximum achievable black stripe width. Placing the diffusion layer behind the black stripes would also result in a loss of quality and definition of the edge of the black stripe. Also, by adding a diffusive element on the viewer side of the black stripe region, the thickness and cost of the screen is increased. The same is true of configurations that use microlens arrays that have a substantial optical orientation along one axis.
Typical projection screens use a single symmetric diffusion layer. However, often a symmetric amount of diffusion is not preferred. For example, in a screen application with the lenticules of a lenticular lens oriented vertically, the lenticules spread light in the horizontal direction and only minor additional diffusion is necessary in the horizontal in order to achieve diffusion of the source image. Whereas, significantly more diffusion may be required in vertical because the lenticular lens does not affect the vertical angles of light. When using a symmetric diffuser, the diffusion is the same in the horizontal and vertical directions. Therefore, when trying to reach a specific horizontal and vertical viewing angle target, it is likely that the viewing angle in the opposite direction is not optimized and this decreases the gain of the screen over the desired viewing angles. Increased control in the viewing angles for diffusers in an imaging material is needed for efficient use of the projected light.
Also, by adding a diffusing region on the viewing side screen, the ambient light reflected is increased. This reduces the image contrast of the display in ambient light conditions. The increased diffusion needed to expand the light in the vertical direction contributes to an increase in the ambient light reflected, thus reducing image contrast. Essentially, one would like to have better control of the amount of diffusion.
The demand for higher resolution displays has also increased the visibility of speckle. “Speckle” is the optical interference effect resulting from the interference of light rays emerging from a scattering element—such as a screen—that are mutually coherent. The viewers' eye integrates this optical effect and sees a visible pattern. Speckle is typically measured by looking at the variation in intensity across a uniformly illuminated screen. “Speckle contrast” is defined as the ratio of the standard deviation of the intensity to the average intensity. A projection system with “high” speckle contrast means that the speckle pattern is more visible than a system with “low” speckle contrast.
Screens that use more than one scattering layer typically use optical adhesives to combine the screen components; or, spherical light scattering particles are added to Fresnel lenses, lenticular lenses, substrates or other elements. This often introduces spurious interfacial reflections at the element interfaces, that reduces the contrast of the screen and adds to the production cost. When the interfaces are slanted or curved, such as the case with Fresnel lenses or lenticular lenses, respectively, the spurious reflections are more significant, and reduces the optical efficiency, and, possibly, reduce image contrast. For instance, a small amount of spherical particles added to a Fresnel lens to reduce speckle contrast can cause a significant amount of the scattered light to totally internal reflect within the Fresnel lens because of the large slant angles on the features of the Fresnel lens. This reduces the speckle contrast at the expense of reducing image contrast and reducing optical efficiency by lowering the screen transmission.
In projection systems, traditional methods for adding a diffusive region (volumetric or surface relief) in the light path before the aperture regions in the contrast enhancing element will increase the apparent size of the apertures and also result in decreased edge definition. This reduces the performance and increases the reflection of ambient light and the resulting backscatter reduces image contrast.